Materials and methods
The materials for the study were soils sampled from the Carbon Action
experiment (Mattila et al., 2022) intensive observation set (Mattila,
2020), where 20 farms test carbon sequestration (cover crops, compost,
grazing practices, intercrop leys and subsoiling). The experiment
started in 2019 and the samples were collected in July 2021. Each farm
had a split field, where one side had carbon farming practices
implemented and the other was held as a continued-normal-practice
control. The samples were collected from 3 GPS located points on each
field from a 10 m radius from the center of the point with a 16 mm soil
corer from a depth of 0-17 cm. Each field had 30 cores collected, which
were pooled, dried at room temperature (fan assisted) and gently sieved
through a 5 mm sieve. As the soil sieving and milling can influence the
results, all soil sample processing was done following established
guidelines (Franzluebbers and Haney, 2018). (Due to an unfortunate
laboratory accident, 5 samples were lost during processing, resulting in
an overall sample amount of n=35.) The sampled soils covered a large
range of soil texture and organic matter: the median clay content was
35% (4-63%) and the OM 6.8 % (2.6-15.5%). The farming systems
covered annual cropping, grass in rotation and perennial pastures.
For CO2 burst analysis, the dry samples were rewetted to
approximately 50 % pore space (i.e. 30 ml of soil and 9 ml of water;
Woodsend lab manual). The sample was placed in a 475 ml container and
sealed with a CO2 measurement cap fitted with a
datalogger (Woodsend IRTH). The CO2 concentration was
measured for 24 h and the increase in CO2 level over
time was converted to mg CO2-C/kg3soil by multiplying with container air space, dividing by sample mass
and converting to mass units using the ideal gas law.
For the redox analysis, soils were also rewetted to 50% pore space
(Husson et al., 2016). The Redox was measured with an Extech RE300
Exstik Platinum oxidation-reduction-potential sensor (platinum
electrode, silver/chloride reference electrode). The flat end ORP sensor
was pressed to the moist soil sample and allowed to stabilize 1-3
minutes, until the ORP reading changed only slowly. Three repeated
measurements were made of the same sample and the average value was
used. The sample pH was measured with a Horiba LAqua Twin pH meter,
using 1:1 ratio of distilled water:soil. The ORP reading was converted
to Eh (mV) by adding the reference electrode voltage (200 mV) and pH
corrected to a relative hydrogen score by the equation
rH2 = Eh/30 + 2 pH. For interpretation, the readings
were compared to suggested norms for ”healthy” soils (Husson, 2013).
For additional interpretation, the soil samples were classified
according to organic matter content, soil clay content, soil structure
(VESS (Ball and Munkholm, 2015)) and type of crop (perennial, annual).
These were collected and published as ongoing monitoring in the Carbon
Action experiment (Mattila and Girz, 2021).
The statistical analysis was done in R programming language (R Core
team, 2022). Correlations between measured variables were calculated
with Pearson’s correlation. Significance between the differences in two
groups was tested with Mann-Whitney U-test (Wilcoxon). The effect size
of carbon farming practices was tested by fitting a linear model to the
data, using each farm as a blocking factor.